Electronic tape measures have been known and available for a number of years. In many such devices, the length measured is determined by mechanically or optically tracking the length of a tape unwound from a rotating take-up reel located within the case. The tape measure has means associated with the rotating take-up reel to cause the generation of a number of electrical pulses corresponding to the length of the extended tape. These pulses are counted and converted to visual form for display.
The counting function may be implemented in the form of an encoder. Such an encoder may be provided on the take-up reel in the form of, for example, mechanical contacts defining multiple rotary switches coupled to the motion of the take-up reel.
The measuring devices of the above described type, although relatively simple in structure and capable of providing measurement reading at precise intervals, tend to be unreliable and inaccurate due to mechanical shock resulting in missed or spurious pulses. Furthermore, these devices do not provide simple means for detection of measurement errors and for the correction of measurement readings.
It has also been known to use the tape blade itself for encoding displacement data and to employ various optical readers to read the visible indicia on the tape blade. The use of photosensors to read visible indicia imprinted on the tape also involves problems, such as provision of sufficient light, as well as problems related to mechanical damage to the tape surface or contamination by dirt, grease and the like.
Some of the known electronic measuring devices employing optical techniques have measurement data imprinted on the tape surface in the form of reflective and non-reflective bar-code elements. However, in order to provide the degree of accuracy normally required for such measuring devices, which should be no less than 1/16 of an inch, both the code on the blade and the optical reader have to be high precision components. The code has to be imprinted with a high degree of precision which is not normally found in the manufacturing of conventional tape rules. This, in turn, renders the manufacturing of such measuring devices unjustifiably expensive. Also, most prior art devices use complex optical technology for reading the coded tapes with optical elements of the high resolution type requiring both exacting assembly and precision printing of the coded tape. Therefore, these measuring devices are expensive and difficult to manufacture while using existing tape rule manufacturing methods.
Furthermore, while optical reading of coded tape is a very reliable measuring method for use in a clean environment, it is more likely to be subject to contamination and mechanical surface damage under normal use, for example, on a construction site rendering the device highly unreliable.